The present invention relates to an acetabular implant for a hip prosthesis, of the type comprising a cup which is designed to receive an insert freely articulated in the cup, and which is equipped with iliac extensions and with an obturator element for bone fixation.
This implant is for first intention use and also for revision, that is to say it can be put into place not only the first time, but also if the bone cavity needs to be altered or improved, in other words if the pathology causes bone defects to appear which require grafts of moderate size. In other words, this implant can be used for revision as long as the cotyloid cavity has not been destroyed by more than 50%.
French patent 93.12.097 (2,710,836) describes an acetabular implant of this type for a total hip prosthesis in which the cup has a spherical shape, which is the geometry generally used for such implants. However, such a configuration is not particularly well adapted to the anatomy of the acetabular cavity of a hip, the upper wall of which protrudes farther than the lower wall. The result of this is a mechanical stability which leaves something to be desired.
In addition, in the known implants, the iliac tabs generally include a rectilinear part directly attached to the opening edge of the cup. The result of this is a lack of adaptation to the anatomy at this site, in particular to the cotyloid brow, which is likely to affect the proper anchoring of the prosthesis.
A great many known implants have a cup with a smooth surface in contact with the wall of the cotyloid cavity, sometimes equipped with anchoring points, so that their mechanical stability may become compromised in the long term. To remedy these shortcomings, it has been proposed, for example, to perforate the wall of the cotyloid cavity (U.S. Pat. No. 3,740,769, Gierman patent 3,205,526). However, experience has shown that these provisions are not entirely satisfactory.
The object of the invention is therefore to make available an acetabular implant designed in such a way as to afford it excellent fixation during and after surgery, both in the medium term and in the long term.
According to the invention, the cup consists of a hemispherical part to which the obturator element is fixed and which is continued by a cylindrical part to which the iliac extensions are fixed.
Thus, the cylindrical part, whose width and angular extent are suitably determined, continues the cup in the zone of the iliac extensions, that is to say in the upper part of the cotyloid cavity. This cylindrical part is thus in contact with the bone wall as far as the edge of the acetabular cavity, with which it ensures better contact than a simply hemispherical cup. In other words, the upper part of the implant closely covers the bone wall, which considerably reduces the risk of dislocation in the extreme angular positions of the associated femoral stem and of the insert articulated in the cup.
According to an advantageous embodiment of the invention, the cylindrical part extends over substantially a half-circumference of an equatorial edge of the hemispherical part and is delimited by a plane inclined on the equatorial plane of the hemispherical part, this inclined plane continuing beyond the cylindrical part via a truncated edge of the hemispherical part situated in the same inclined plane.
The advantage of the truncation of the spherical part thus realized in the lower zone of the implant lies in the fact that, in the extreme position of the insert in internal flexion-rotation or in external extension-rotation, it prevents the escape of the insert from the cup by a cam effect. This is because the neck of the femoral stem can no longer come into abutment on the lower opening edge of the cup and thereby risk expulsion of the insert.
In the extreme flexion position, the neck of the femoral stem does not risk coming into abutment against the cylindrical part of the cup, the width of which is suitably chosen for this purpose.
Indeed, this geometry of the acetabular implant greatly reduces the risk of dislocation in the two opposite extreme positions of the insert and of the associated femoral stem, while at the same time promoting excellent angular clearance. Moreover, the geometry of the cup enables it to make maximum provision for the anatomy of the acetabular cavity.
According to another characteristic of the invention, the iliac extensions comprise a rectilinear end attached to the edge of the cup, continued by a curved part whose curvature is adapted to the cotyloid brow of an acetabular cavity, a rectilinear part and then an incurved part continuing said curved part; lastly, a final rectilinear part which is attached to the incurved part and in which a hole is formed for passage of a bone anchoring screw.
Advantageously, the two extreme rectilinear parts delimit between themselves an angle of about 45 degrees preferably.
This geometry allows the iliac extensions to adapt closely to the bone anatomy at this site, near the edge of the acetabular cavity, which edge is itself modeled if necessary.
According to another particular feature of the invention, a macrostructure with raised parts promoting primary anchoring and bone integration is machined on the outer surface of the cup situated substantially between said inclined plane delimiting the opening of the cup and a spherical cap delimiting the bottom of the cup, and this macrostructure and the cap are coated with calcium hydroxyapatite.
This macrostructure, the geometry of which is suitably chosen so as to be neither too smooth nor too rough, permits better primary and long-term anchoring of the cup by means of bone regrowth, thereby giving the implant its long-term stability.